Added function to check integer argument

* src/git2r_error.c/h (check_integer_arg): Added function.
Added stash test cases

* tests/stash.R: Updated with stash test cases
Added method to drop stash

* R/stash.r (drop_stash): Added generic drop_stash and method for S4
  classes git_repository and git_stash.

* man/drop_stash-methods.Rd: Added documentation

* src/git2r_stash.c/h (drop_stash): Added C code to drop stash in
  repository.

* src/git2r.c: Added "drop_stash" to callMethods

* NAMESPACE: Updated